A navigation system performs travel guidance for enabling a user to easily and quickly reach the selected destination. A typical example is a vehicle navigation system where a vehicle is equipped with a navigation function to guide a driver to a destination through a calculated route. Such a navigation system detects the position of the user's vehicle, and reads out map data pertaining to an area at the current vehicle position from a data storage medium, for example, a CD-ROM (compact disc read-only memory), a DVD (digital versatile disc), or a hard disc, or from a remote server.
When a destination is specified by a user, the navigation system determines a route to the destination based on, for example, the shortest way to reach the destination, the route using many free ways as possible or the route without using toll road, and the like. After determining the calculated route, the navigation system starts the route guidance operation to guide the user along the calculated route to the destination. Typically, the navigation system shows the intersection to show the next turn and a direction of the turn. Such route guidance by the navigation system is also accompanied by voice instructions.
The navigation system displays a map image on a monitor screen on which the calculated route to the destination is illustrated. The user is able to scroll the map image to see desired locations of the map image. FIGS. 1A and 1B show display examples when such an operation of map scrolling is conducted on the navigation system. In this example, FIG. 1A shows a display screen 81 which includes an initial map image, and FIG. 2B shows the display screen 81 which includes a map image coming after scrolling the map image of FIG. 1A toward a down-right direction which is indicated by an arrow A.
The map images in FIGS. 1A and 1B show several roads and two icons 71a and 71b indicating POIs (points of interest), representing a restaurant and a gas station, respectively. The description indicator 75 at the top of the screen typically indicates a road name that intersecting with a current road on which a vehicle, which is indicated by a vehicle position VP, is running to indicate the next turn on the calculated route. The user can scroll the map image by placing a finger on the display and dragging the image or by operating a scroll key on the navigation system or a remote controller. The scroll keys may be configured by eight directional arrow buttons or a joystick key on a remote controller similar to the one commonly used in electric game devices.
In the scrolling operation, the currently available navigation systems utilize either a fixed speed scroll operation or a stepped scroll speed operation. FIGS. 2A and 2B are schematic diagrams showing the fixed scroll speed operation and the stepped scroll speed operation, respectively. In the fixed scroll method shown in FIG. 2A, the scroll speed remains constant as the time changes throughout the operation. In the stepped scroll method shown in FIG. 2B, the scroll speed incrementally changes along with the time.
Thus, in the stepped scroll speed method, as the user scroll the map on the monitor screen, the speed of scrolling accelerates along with the time. The stepped scroll speed method has the advantage that the user can browse the map at a faster rate to save time and reduce distraction. However, in either method of scroll operation, during the scroll operation, the navigation system may not be able to redraw map image with sufficient detail.
Namely, depending on performance factors, such as computation speed, complexity of the map, and available resources, etc., the navigation system may have to simply show empty un-drawn spaces (blank space). As a result, the navigation system fails to provide useful information for scrolling the image to find a desired location on the map image. Therefore, there is a need of a new method and apparatus for scrolling the map image with an optimum size and speed appropriate to currently available resources of the navigation system.